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This work presents wavelength measurements of axial instability waves along the surface of a liquid jet operated within the atomization regime. The surface waves appear in the near exit region of the initially laminar jet. The measurements show that the waves stretch as they are convected downstream before the flow becomes turbulent. A set of experiments using three different cylindrical nozzles of length-to-diameter ratios of one, five and ten are performed at jet velocities ranging between 10 and 40 m/s. These sets of tests allow variations in flow Reynolds number, based on the momentum thickness at the exit of the nozzle, between 150 and 750. The measurements provide statistically validated wavelength measurements as a function of the axial coordinate. These measurements are then compared with three stability models and are found to support the hypothesis that the most important mechanism responsible for the presence of the instability waves is a pocket of absolute instability present near the exit of the nozzle. This pocket is responsible for amplifying disturbances of a given frequency as well as feeding them upstream into the flow.

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Low Swirl Injector (LSI) technology is a lean premixed combustion method that is being developed for fuel-flexible gas turbines. The objective of this study is to characterize the fuel effects and influences of combustor geometry on the LSI’s overall acoustic signatures and flowfields. The experiments consist of 24 flames at atmospheric condition with bulk flows ranging between 10 and 18 m/s. The flames burn CH4 (at φ = 0.6 & 0.7) and a blend of 90% H2 - 10% CH4 by volume (at φ = 0.35 & 0.4). Two combustor configurations are used, consisting of a cylindrical chamber with and without a divergent quarl at the dump plane. The data consist of pressure spectral distributions at five positions within the system and 2D flowfield information measured by Particle Imaging Velocimetry (PIV). The results show that acoustic oscillations increase with U0 and φ. However, the levels in the 90% H2 flames are significantly higher than in the CH4 flames. For both fuels, the use of the quarl reduces the fluctuating pressures in the combustion chamber by up to a factor of 7. The PIV results suggest this to be a consequence of the quarl restricting the formation of large vortices in the outer shear layer. A Generalized Instability Model (GIM) was applied to analyze the acoustic response of baseline flames for each of the two fuels. The measured frequencies and the stability trends for these two cases are predicted and the triggered acoustic mode shapes identified.